This invention relates to reconstituted mica materials, reconstituted mica prepreg materials, reconstituted mica products and insulated coils.
As reconstituted mica, there has generally been used so-called fired reconstituted mica produced by heating muscovite to about 800.degree. C. for dehydration, disintegrating expanded muscovite with stirring in water or by a jet stream of water to give a slurry of mica flakes or scales having a particle size of as small as about 0.2 to 1.0 mm, and sheet forming by using the resulting slurry containing mica flakes to give mica foils. There has also been used so-called soft reconstituted mica produced by disintegrating phlogopite in water to give a slurry of mica flakes having a particle size of as small as about 0.2 to 2.0 mm, and sheet forming by using said slurry to give mica foils.
Fired mica cannot be disintegrated industrially to thin flakes of 5 to 10 mm square. Moreover, since fine cracks are produced in mica flakes or scales after leaving the water of crystallization of mica, strength of mica scales is low and an effect as a reinforcing material is small. Further, fired reconstituted mica has a density of as low as 1.3 to 1.5 g/cm.sup.3 and a large number of voids in the mica foils, so that a large amount of thermosetting resin composition is necessary for giving, for example, prepreg materials, which results in giving products having a larger resin content with poor mechanical and electrical properties when molded with heating under pressure. In order to give heat resistant reconstituted mica, when a large amount of heat resistant thermosetting resin composition is used, there takes place lowering in heat resistance or remarkable deterioration in properties in a long period of time.
On one hand, phlogopite is slightly superior to muscovite in heat resistance but inferior to muscovite in electrical properties, so that it cannot be used for insulating high voltage machines and instruments such as a dynamo.
On the other hand, in the case of disintegrated unfired mica particles, it is possible to lower the amount of binder and the like at the time of processing them to reconstituted mica products compared with disintegrated fired reconstituted mica particles, but they are insufficient in mechanical and electrical properties compared with splitting mica products.
In order to improve the above-mentioned defects, unfired reconstituted mica under a trade name of Micanite II has been devloped by U.S. SAMICA Corp. Micanite II is produced by disintegrating unfired muscovite flakes in water by using ultrasonic energy. Its particle size distribution is listed in Table 1 together with the particle size distribution of conventionally used reconstituted mica.
TABLE 1 ______________________________________ (% by weight) Average particle Conventional. size (mm) Micanite II reconstituted mica ______________________________________ 3.4 15 0 2.7 19 1 1.7 22 2 0.97 33 33 0.38 9 32 0.18 2 23 0.10 0 9 ______________________________________ (from Catalogue of U.S. SAMICA Corp.)
Micanite II is unfired reconstituted mica but has tensile strength of as strong as 0.7 to 1.3 kgf/mm.sup.2 without using a binder and the resulting mica foil has a density of as high as 1.6 to 1.7 g/cm.sup.3. But Micanite II has some defects caused by the particle size distribution as shown in Table 1. That is, since larger particles are used in Micanite II, reinforcing effect by the mica particles is great, which results in improving mechanical properties of processed mica products, but, on the other hand, since the closest packing can not be attained by using larger mica particles, deterioration of properties, particularly electrical properties after a long period of time is more remarkable than shown by splitting mica products.
Splitting mica products mean mica cleaved or delaminated by hand and having a diameter of about 50 mm or more and 30 to 50 .mu. thick. Since the size of this mica is so large that it cannot be sheet formed by using a wet type paper machine unlike reconstituted mica, it is sheet formed by hand or using a semiautomatic machine in practical production. Therefore, the production cost of splitting mica products becomes higher and finally the splitting mica products are more expensive than the reconstituted mica products because the former is not suitable for a large scale production by using a machine, and the like. Properties of the splitting mica products are excellent even after long time deterioration, since larger scales than a conventional reconstituted mica are used and the larger scale function as insulator even after the deterioration of binder.
In order to apply features of both the splitting mica products and the reconstituted mica products, combination of the both is proposed, for example, in Japanese Patent Appln. Kokai (Laid-Open) No. 58500/78, wherein both sides of delaminated mica sheet are bound with reconstituted mica sheets. The resulting product is improved in the accuracy of the thickness compared with the splitting mica product but rather lowered in properties after deterioration for a long period of time compared with the laminated mica product alone.
Improvement of the defects mentioned above has long been desired.
In the case of insulated electrical coils, they are produced by a process comprising winding splitting mica tape around a conductor, impregnating the wound tape with a thermosetting resin composition such as an epoxy resin composition, an unsaturated polyester resin under vacuum, and curing the resin, a so-called prepreg process comprising winding fired mica prepreg tape around a conductor, and molding with heating under pressure, or the like. The prepreg process is superior in initial electrical properties but not so good in other properties, while the vacuum impregnation process using splitting mica tape is superior in initial mechanical properties and electrical properties after deterioration by voltage endurance test but not so good in other properties.
When reconstituted mica is used, there has been no insulated coil having the same or higher properties compared with the case of using the splitting mica. An insulated coil having the same electrical properties as in the case of using the splitting mica is disclosed in Japanese Paten Appln. Kokoku (Post-Exam Publn) No. 20264/75. But the process disclosed therein has defects in that there are necessary complicated procedures such as wetting raw mica blocks with an aqueous solution of hydrofluoric acid or hydrogen choride, and also necessary for treating waste water. Further the resulting insulated coil is not so sufficient in mechanical properties since fired mica is used.